Four hours -- that's the time Joomla website owners had to apply a patch recently before attackers started to exploit the flaw it fixed. Those who still haven't updated their websites are likely to find them compromised.

On Thursday, the developers of Joomla released version 3.4.5 of the popular content management system in order to fix an SQL injection vulnerability that allows attackers to gain administrative privileges by hijacking an active administrator session.

Less than four hours after the update's release and the publishing of a technical overview by security researchers at Trustwave, attackers were already exploiting the flaw. Web security firm Sucuri said it saw attacks against two of its customers who operate very popular Joomla-based websites.

"What is very scary to think is that neither of these sites were patched at the time," Sucuri's CTO Daniel Cid said in a blog post Monday. "The disclosure happened on a Thursday afternoon (evening in Europe), when many webmasters were already off for the day."

Within 24 hours there were already Internet-wide scans probing for the flaw and the number of attacks continued to increase over the weekend. On Monday, Sucuri recorded 12,000 exploitation attempts against the Joomla sites of its customers.

Based on this incident, the administrator of an average website has a time window of less than 24 hours to patch following a serious vulnerability disclosure. If the website is a highly popular one, the reaction time should be within a few hours.

These fast attack times make the use of Web application firewalls, intrusion detection systems, access control policies and proper logging increasingly important in defending Web properties.